Ethane/Ethylene Separations in Flexible Diamondoid Coordination Networks via an Ethane-Induced Gate-Opening Mechanism.

Separating ethane (C 2 H 6 ) from ethylene (C 2 H 4 ) is an essential and energy-intensive process in the chemical industry. Here, we report two flexible diamondoid coordination networks, X-dia-1-Ni and X-dia-1-Ni 0.89 Co 0.11 , that exhibit gate-opening between narrow-pore (NP) and large-pore (LP) phases for C 2 H 6 , but not for C 2 H 4 . X-dia-1-Ni 0.89 Co 0.11 thereby exhibited a type F-IV isotherm at 273 K with no C 2 H 6 uptake and a high uptake (111 cm 3 g -1 , 1 atm) for the NP and LP phases, respectively. Conversely, the LP phase exhibited a low uptake of C 2 H 4 (12.2 cm 3 g -1 ). This C 2 H 6 /C 2 H 4 uptake ratio of 9.1 for X-dia-1-Ni 0.89 Co 0.11 far surpassed those of previously reported physisorbents, many of which are C 2 H 4 -selective. In situ variable-pressure X-ray diffraction and modeling studies provided insight into the abrupt C 2 H 6 -induced structural NP to LP transformation. The promise of pure gas isotherms and, more generally, flexible coordination networks for gas separations was validated by dynamic breakthrough studies, which afforded high-purity (99.9%) C 2 H 4 in one step.